The invention relates to a method for determining the thickness of at least one layer provided on a substrate.
When applying layers to a substrate, it is necessary that a certain layer thickness must be created or maintained. This is particularly true for the manufacture of storage media, in particular optical data storage media such as CDs, CD-Rs, DVDs, CD-RWs, DVD-RWs, MOs, and other data storage media to which different layers, such as information carrier layers, protective layers, or reflection layers, are applied. Known for measuring the layer thicknesses for quality control and during the production process are various layer thickness measurement processes, but these processes cannot be employed on grooved substrates. For determining groove geometry, reflection and/or transmission are measured for various orders of diffraction. However, such measurement processes with higher orders of diffraction of the light require more complex measurement equipment and calibration measures. It is not possible, or it is only possible with limited accuracy, to measure the layer thicknesses if the substrate itself has structures such as troughs that in optical storage media, for instance, are called grooves and that are required for writing, reading, and/or deleting data.
The object of the invention is therefore to create a method that provide a simple and reliable measurement process, provides reliable measurements for its control and regulation, in particular even during the layer application process, and also provides reliable measurements for substrates with structures contained therein or formed on the surface thereof, and that makes it possible to determine the structure geometry.
The object set forth is achieved in a method for determining the thickness of at least one layer provided on a substrate by measuring reflection and/or transmission light intensity values of zero order of diffraction as a function of the wavelength and by calculating the reflection and/or transmission light intensity values using an iteration model that depends on the individual layer parameters, whereby the layer parameters are modified to effect correspondence between the measured values and the calculated values, and whereby the substrates have geometric structures, the geometric dimensions of which are used as additional parameters of the iteration model.
In contrast to the conventional measurement processes, the inventive measures make it possible to correctly determine layer thicknesses, even of substrates with structures, as is the case, for instance, with CD blanks with grooves to which various layers must be applied, each layer having subsequently determined layer thicknesses, in order to manufacture optical data storage media. In addition, the method, and in particular the measurement device provided therefor, is very simple, because in the inventive method it is only necessary to measure the reflection/transmission light intensity values of the zero order of diffraction. This enables a very simple measurement arrangement that in particular can be used in the manufacturing process of, for instance, optical data carriers. Furthermore, calibrating the inventive method, in which only light values of zero order of diffraction are measured, is substantially less complex than in conventional processes. It is therefore possible to perform a wide variety of measurement processes with a single measurement device, as will be explained in more detail in the following.
In particular sample preparations are also no longer required with the inventive method, since the thickness of layers that are or will be applied to substrates with structures can be determined accurately and reliably. The process is therefore also particularly employable even during the manufacturing process, that is, it can be used in-line, since thickness measurements and controls are possible directly on the produced products, such as optical data storage media.
Thus, in accordance with the inventive method, the transmission light intensity values in reflection and/or transmission are measured spectrally, that is, as a function of the wavelength. These measurement values are then evaluated using optical calculations. This approach is to calculate the reflection and/or transmission light intensity values from optical models for a layer system and to vary in every calculation step the layer thicknesses and/or the spectral material parameters, e.g., the refraction indices (n) and/or the absorption indices (k) until the minimum deviation between measurement and calculation is achieved. Thus a layer thickness control is possible, and therefore monitoring of the optical properties of a layer system on a substrate with structures is possible, for instance for CDs with grooves.
In accordance with all of the preferred embodiments of the invention, interferences that occur due to the geometric structures are used for additional parameters. The inventive method in accordance with this embodiment is based on calculating inphase overlap of electromagnetic partial waves on the structures, for instance, on the grooves of the substrate or blank, and therefore on the interferences of these partial waves. By varying the parameters, the width and depth of the structures, for instance the channels or grooves, can be included in the measurement process. It is not possible to achieve optical consistency between measurements and calculation unless the structure parameters, the width, the depth, and/or the intervals between the structures or channels is consistent with that of the channels on the measured sample.
Due to the spectral dependence of the changes in the reflection and/or transmission light intensity values based on the structures, that is, the channels or grooves, compared to the light intensity values without structures, the information on the structure parameters, for instance the width, depth, and interval between channels or grooves, is already contained in the light intensity values of zero order of diffraction.
In accordance with a particularly advantageous embodiment of the invention, the geometric dimensions of the structures are determined, that is, for instance, the depth, width, and/or interval between channels or grooves. These geometric dimensions can be calculated preferably at the same time as the determination of the layer thicknesses. In this manner it is possible to monitor and control the quality of the molding process, for instance during production of optical storage media. For the special case in which no layers are applied to the substrate, that is, the layer thicknesses are zero, the measurement values for the geometric dimensions of the structures result, whereby it is thus possible to perform localized measurements of these geometric structures over the entire substrate surface in order thus to rapidly detect production errors during the production of substrates, for instance during injection molding of CD substrates.
In a very advantageous additional embodiment of the invention, the calculated data for the layer thicknesses and/or geometric structures are used for regulating production processes for applying at least one layer to a substrate and/or for developing substrate structures. The inventive method is particularly well-suited for in-line employment since in the inventive method only the light intensities of the zero order of diffraction need to be measured, and therefore neither the measurement apparatus nor the calibration is very complex, and since a wide variety of measurement options are possible with a single measurement apparatus, for instance layer thickness measurements alone or in combination with the geometric dimensions of the substrate structures, or the geometric dimensions of the substrate structures alone. This means that during production of the optical data carriers, for instance during the manufacture of substrates, when substrate structures are being developed, or during coating of the substrate with pre-specified layer thicknesses, the inventive method can be directly employed and can be used for regulating and controlling the production process. Thus the data calculated with the inventive method for the layer thicknesses and/or the substrate structure are used for subsequently determining the values for correcting variables in the production facility. In this manner the course of production can be automatically adjusted, controlled, and regulated.
In the case of production of optical data carriers such as CDs, the inventive methods are used directly on the produced products to determine the actual values. These actual values are then directly used to provide new correcting variables, such as sputter times, sputter rates, as well as pressures, temperatures, and gas flows during the production process in the production facility. The inventive method makes it possible to input target values into the production process, which values are then achieved via the closed loop. Since in the case of production of optical data media, in addition to the layer thicknesses, the groove geometries are also determined with the inventive method, in this manner the quality of the substrates or blanks with the geometric structures can also be controlled, that is the quality of the molding process can be controlled.
A special case for the inventive process is when the substrates do not have a geometric structure, that is, only the layer thicknesses are measured and/or regulated.
Preferably the substrates are blanks for data storage media such as CDs, whereby the geometric structures are formed as channels or grooves in the blank, and at least one information-carrying layer is applied to these blanks. The information-carrying layer is a preferably a metal alloy that can be modified by the energy from a beam of light between two phases. The information-carrying layer is preferably formed between two buffer layers.